IP Multimedia (IPMM) services provide a dynamic combination of voice, video, messaging, data, etc. within the same session. By growing the numbers of basic applications and the media which it is possible to combine, the number of services offered to the end users will grow, and the inter-personal communication experience will be enriched. This will lead to a new generation of personalised, rich multimedia communication services, including so-called “combinational IP Multimedia” services which are considered in more detail below.
IP Multimedia Subsystem (IMS) is the technology defined by the Third Generation Partnership Project (3GPP) to provide IP Multimedia services over 3G mobile communication networks (3GPP TS 23.228 and TS 24.229 Release 5 and Release 6). IMS provides key features to enrich the end-user person-to-person communication experience through the integration and interaction of services. IMS allows new rich person-to-person (client-to-client) as well as person-to-content (client-to-server) communications over an IP-based network. The IMS makes use of the Session Initiation Protocol (SIP) to set up and control calls or sessions between user terminals (or user terminals and web servers). The Session Description Protocol (SDP), carried by SIP signalling, is used to describe and negotiate the media components of the session. Others protocols are used for media transmission and control, such as Real-time Transport Protocol and Real-time Transport Control Protocol (RTP/RTCP), Message Session Relay Protocol (MSRP), Hyper Text Transfer Protocol (HTTP). IMS requires an access network which would typically be a 2G/3G General Packet Radio Service (GPRS)/Packet Switched (PS) network, but which might be some other access network such as fixed broadband or WiFi. FIG. 1 illustrates schematically how the IMS fits into the mobile network architecture in the case of a GPRS/PS access network.
The TISPAN working group of the European Telecommunications Standards Institute (ETSI) is currently working on a proposal for the Next Generation Network (NON) for fixed networks based upon IMS. As part of this project, consideration will be given to a so-called Home IMS Gateway (HIG) which will allow non-IMS terminals to access IMS services. It is expected that the HIG will find applications in the home and small office environments where users might wish to access IMS services using a number of non-IMS enabled terminals which may or may not be SIP terminals. Examples of non-IMS but SIP enabled terminals are SIP telephones and PCs, whilst examples of non-IMS terminals which do not have SIP functionality are legacy telephones including DECT telephones. The HIG will include a SIP gateway in order to handle interoperability issues (e.g. conversion between SIP and other signalling protocols required by user equipment).
The current proposal for a HIG conflicts to some extent with the current access security requirements for IMS as defined in the 3GPP Technical Specification 33.203. These requirements specify that each terminal accessing the IMS have access to an IMS SIM (ISIM) application. The ISIM stores subscriber identity information, as well as other information required to authenticate a subscriber to a network operator and to authorise access to network enabled services. ISIM applications are provided on Universal Integrated Circuit Cards (analogous to today's GSM SIM cards). Of course, non IMS terminals such as legacy equipment will not have the necessary UICC card reader or IMS functionality.